Coronal observations from the soft X-ray telescope on Yohkoh

0273-I 177/w J6.00 + 0.00 (‘q~yfight Co1993 COSPAR

Ad% SJXJCY Hes. Vof. 14, No. 4, pp. (4)3-(4)l I, 1994 Printed in Great Britm. All rights reserved.

CORONAL OBSERVATIONS FROM THE SOFT X-RAY TELESCOPE ON YOHKOH K. T. Strong* and Yohkoh Team** * L&heed

Solar and Astrophysics Laboratory,

Palo Alto, CA 9430J, 1J.S.A.

** Institute for Space and Astronuuticul Studies. Mullurd Spucc S(Gt(x* Lclborutory, National Astronomicul Obsrrvutor~v of Jupan, Nuvul Rl*.\c~,rr~~h Laboratory, Corporation,

Rutherford und Appleton Laboratory,

Solar Physics Rcsctrrc,h

Stunford University, Univer,sity of Culifomiu

University of Hawaii,

Berkeley.

University of Tokyo

ABSTRACT The Yohkoh payload includes a Soft X-ray Telescope @XT) which is capable of taking high-resolution (2.5 arcsec) images of the Sun through several X-ray and optical filters. A CCD camem provides images with a lo\\ background, and large dynamic range and at a rapid cadence (>2 s), which enables the SXT to observe the effec.14 of flares while continuing to observe the fainter quiet-Sun features. We present the observations from the SXT that illustrate how the faint coronal loops that make up the complex structure of the quiet Sun evolve on a variety of timescales from seconds to months. We describe a variety of coronal structures, such as coronal holes, bright points, the large-scale quiet coronal loops, and active regions, to show the wide mnge of opportunities that Yohkoh presents to further our understanding of the solar corona. INTRODUCTION has been in orbit for over a year observing the solar corona almost continuously. Its main objective is to study the impulsive phase of flares. However, the Sun is in the declining phase of cycle 22, following solar maximum late in 1989, so there are long periods when the Sun has been relatively inactive when Yohkoh can concentrate on quiet-Sun objectives. The Soft X-Ray Telescope (SXT) has taken nearly a million images of the corona, producing a wealth of data for the solar community to sift through and improve our understanding of this hightemperature part of the solar atmosphere. It has been able to observe the structure and evolution of active regions, the quiet Sun, and coronal holes on timescales from a few seconds to months. Yohkoh

Vaiana et al. /1,2/ described many different types of features from the sounding rocket and Sk$uh X-ray images. They dispelled the idea of a quiet homogeneous corona by showing it to be highly structured as a result of the hot plasma outlining the magnetic field. The majority of the magnetic field is closed rather than open and connected to the heliospheric field. The most intensely emitting and densest population of loops, active regions, were found to be above sunspot groups. Active regions were often seen to be joined by large-scale loops, interconnecting them into a complex network of magnetic fields, whereas the large-scale loops often seemed to be anchored in the areas corresponding to the chromospheric network. Krieger et al. /3/ pointed out the correspondence of X-ray bright points (XBPs) to small magnetic bipoles and suggested that they are small active regions. Coronal holes were believed to be the only areas of open magnetic field and the source of the solar wind, on the basis of their low Ievcl of X-ray emission and lower temperatures. Long dark channels seemed to coincide with the location of Ha tilerments. As Skylab was launched late in the decay of the solar cycle and had a limited supply of film, it saw few flares antI was unable to take rapid or extensive sequences of images of them. Further, although the resolution in soft X-my\ was good (~2 arcsec) near the optical axis, it rapidly degraded to about 10 arcsec near the limb. Also, there was ;I high degree of scatter from the mirror, which tended to mask out the fainter details of such events, and film h:iq :I relatively narrow linear dynamic range. The SXT was designed to overcome many of rhese limitations, whi:Il makes it an exciting instrument to observe the evolution of quiet-Sun phenomena as well :I\ il;irrs at a hi2ht.r C’.I dence and with more extended coverage than has hczn possible to date. GENERAL CHARACTERISTICS Ofi THE CORONA Figure I shows a mosaic of several SXT images made a~ different Yohkoh pointin+ bo rh;u us ;Irc ;II+ to see t’;lr

(4)3

(4)4

K.T. Strong and Yohkoh Team

Figure 1: A mosaic of several SXT images taken at quarter resolution in the A1MgMn filter at a range of exposures on 3 March 1993. This is a logarithmic representation of the coronal emission. Note the extent of the corona above the solar limb particularly in the NE quadrant of the image and the large-scale interconnections both to the northern and southern hemispheres. At the east limb of the Sun the bright area stretching about 5 arcmin above the limb represents the post flare loops formed following an M5 flare that occurred some 5 hours before. above the solar limb. Soft X-ray emission is plainly visible out to at least a solar radius above the limb. Some of the emission clearly has closed-loop structure. The-scaie height of the corona at 3 MK is about 2 x 101° cm. We can see by inspection that the corona has many different scale heights, the highest not always corresponding to the hottest and brightest active regions on the Sun. This is indeed curious as SXT is not very sensitive to temperatures below I - - 2 MK and one would not expect that the quiet corona to have temperatures much higher than 3 MK. Hence this makes a very narrow range of temperature and so one would expect a narrow range of scale heights to be observed in the quiet Sun. So, is our understanding of the coronal temperature at fault (see discussion below), or are some of our assumptions about the scale of the corona wrong? First, the scale height is calculated on the basis of the assumption that the coronal plasma is in hydrodynamic equilibrium, but the data do not seem to support that idea. As can be seen from any time sequence of SXT images, the corona is continuously in motion, appearing to expel coronal plasma and magnetic field, which would be consistent with the corona not being in equilibrium. Uchida et al. /4/ describe this new coronal phenomenon and speculate whether the magnetic field can be expelled into the heliosphere and take any significant amounts of plasma with it, thus contributing to the solar wind. This may explain many of the density, magnetic field, and abundance anomalies observed by in situ measurements of the solar wind. The observed velocities are typically a few tens of kilometers per second, which is significantly lower than the escape velocity from the Sun, but it may not need a ballistic velocity if the forces that initiated the expulsion continue to push the field lines outward, taking the plasma with it.

Soft X-myTelescope

(4)5

Figure 2: A selection of coronal structures observed by SXT. The bar inset in each image represents 1 arcmin on the Sun (about 44,000 km) to give an idea of comparative scale. Particularly striking are the helmet-like or cusped loops shown in (a), (b), and (e). (c) and (f) show examples of X-ray jets. (d) illustrates a pair of flaring loops. (g) an east-west slice across the active latitudes, showing a high degree on connectivity in the magnetic fields MAGNETIC LOOPS As discussed above, Skylab showed that the fundamental building block of the corona is the magnetic field which produces an intricate, interwoven pattern of loops. However, the SXT is giving us a new perspective on loop-like sn'uctures in the corona from the point of view of their evolution and stability. Acton /5/ describes the menagerie of loop-like structures (see Figure 2) that SXT has observed so far. While we see all the types of structures defined by Skylab, there is an interesting and relatively common structure that seems to have gone undetected until Yohkoh; the cusped loop (Figure 2a, b, and e). The structure of this seems to be very like a small-scale helmet streamer seen in coronagraph images, but instead of being open or connected to the large-scale field (e.g., Figure 2a) they often are connected back to the surface of the Sun and to another similar structure by a loop-like structure (Figure 2e). The loop joining them is sometimes highly kinked or sheared. Such structures could be examples of current sheets emitting sufficient X-ray radiation to be visible. In Figure 2b there seems to be a twisted pair of loops. Comparing the two such features visible in Figure 2e, the cusped loop to the west (right) is bright at the cusp but the one to the east is bright at the foot points[ The sizes of loops have a large range from the smallest, which are clearly unresolved by SXT such as those seen in XBPs /6,7/ (which appear to join tiny bipoles), to the sort of global restructuring seen in the 12 November 1991 event described by Tsuneta et al./8/. At the SXT highest resolution (2.45-arcsec pixels), an unresolved structure would correspond to at most 3,500 kin. In this event a large arcade of loops in the northwest erupts, filling in an area of quiet Sun with a massive faint arcade of loops that persists for at least a solar rotation. The eruption propagates westward along the arcade at a speed of about 100 krn/s. It overlies a polar-crown filament, which disappeared at the time of this event and He I 10830 A showed large-scale, faint enhanced ribbons along the footpoints of the loops. The ribbons were about 500,000 km long and were separated by about 480,000. The loops were at least semicircular (and may even have been more elongated), so that would put their height at about the same scale, and so their length was at least 1,500,000 kin. Apart from transient activity, there seem to be many large-scale magnetic loops interconnecting active regions and in the quiet Sun (see Figure 1). Most of these loops stretch from one active region to another or connect from an active region to quiet-Sun areas (although we have not as yet attempted to align these with the network structure to test Vaiana's idea/2/. One of the most striking aspects on a careful inspection of these types of loops in an SXT image is the apparent uniform cross-section that most single identifiable loops seem to have. Klimchnk e t a / . / 9 / have investigated the variation of loop cross sections along the length of ten coronal loops and shown that, within the resolution of SXT, that there is no appreciable change of the loop cross-section as a function of its length. This

(4)6

K.T. Su'ong~u]dYohkoh Tc~un

Figure 3: Synoptic Maps of the Sun taken by SXT for Carrington Rotations 1847, 1857 and 1858. These were compiled by taking a vertical slice at the central meridian of every image taken in SXT's thin aluminium filter dttring each rotation and mosaicing them together. means either that the expansion is on a much smaller scale than was previously thought and so SXT cannot resolve it or that the loops have some containing force that keep the cross-section constant along their lengths (such ::~, currents). Martens and Gomez /10/ find that the scale sizes of structures in SXT images follows a power law frequency distribution with a slope of about 3. This seems to be a universal relation as a similar analysis of other data types give similar results. On longer timescales one can examine the stability of large-scale structures such as active regions and coronal holes. This is best achieved by looking at synoptic maps of successive rotations as shown in Figure 3 (G. Slater, private comm.). Figure 3a shows the distribution of X-ray flux for Carrington rotation 1857. This image was built up by taking the entire column of pixels centered on the central meridian of the Sun of all of the Al1200/~ halfresolution (5 arcsec) full-disk images taken by SXT during that period. The width of the swath taken corresponded to the time gap between successive images. Contrast it to the next rotation (1858). While there are significant differences in detail, the general features remain the same, indicating that most of the large-scale structures are dynamically stable on timescaies of a month. In Figure 3c we show a synoptic map from 9 months earlier (1848). The first striking thing is how much more active the Sun was in that time frame, This can be graphically illustrated by considering the Sun as a star (Figure 4 - M. Morrison, private comm.), which shows the integrated flux in the 5 - - 45 ,~ band from SXT full-disk images for the whole Yohkoh mission so far. Note how the curve dips at the end of January 1992, so that in less than a fortnight the general solar flux is halved. The quiet coronal can show large-scale enhancements (flares?) on timescales of minutes to hours. These are most often associated with filament disappearances. While most of them do not show up on the GOES X-ray light curves, the high sensitivity and imaging capabilities of SXT make it possible to pick out these often exquisitely beautiful structures. See Figure 5 for an event observed in the southeastern quarerant of the Sun on 26 February 1992 at about 20:00 UT. Note how the region appears featureless and then within an hour an extended structure appears alonl~ the boundary of a coronal hole channel. It slowly evolves into a distinct arcade over the next 5

Soft X-rayTelescope

(4)7

Figure 4: A time history of the integrated soft X-ray emission from the Sun during the first year of Yohkoh operations. Note how the X-ray output of the Sun has dropped significantly in that time corresponding to a similar decline in the magnetic flux on the Sun (K. Harvey, private communicationJ.

hours, fading away to return to the pre-flarc quiet coronal intensity levels x~ithin a few hours. It is worth noting the complexity of the magnetic field lines ill this structure. The loops seem lwisted and interwoven in a complex weave of magnetic field, combining a mixt,re of large-scale and small-scale ,u:lgnctic interconnections. This is typical of many such events seen by SXT/t 1/. On smaller scales, within active regions there are often sustained periods of short-li~cd brightenings/12/. These seem to be most common in the most active, growing regions and seem to be the result of low-energy, limited reconnections between two or more loops. It is interesting to speculate whether these are the same sort of events that Lin et al. /13/reported as "microflares" from a balloon-borne hard X-ray spcctrometer. The many small events seen by Yohkoh in active regions and a similar phenomenon in X-ray bright points/6/(see XBP discussion below) may well explain the hard X-ray observations. We plan to compare the data with the highly sensitive observations from the Compton Gamma-Ray Observatory to check this hypothesis. They do not seem frequent enough (1 - - 20 per hour in a given active region) or spread evenly enough throughout a region to provide the energy to heat the corona or the whole active region. PLASMA CONDITIONS Armed with some knowledge of the types of geometries that we are dealing with, the next step is to investigate the typical conditions that exist in the corona. In particular, it is important to determine the temperature and emission measure of the coronal plasma because these parameters set the energetics and heating requirements for the corona. Typically the Skylab X-ray instruments found temperatures in active regions of about 2.5 (+1) 1 MK 114/. 1Note the parentheticalvaluehererepresentsthe uncertaintyon the valuenot the range

(4)8

K.T. Strongand YohkohTeam

Figure 5: An example of a large-scale transient structure seen in the quiet corona. This arcade brightened probably as the result of the eruption of a polar crown filament on 26 February 1992. The Solar Maximum Mission (SMM) X-ray Polychromator (XRP), with significantly more sensitive temperature diagnostics from X-ray emission line ratios, found temperatures of about 3 (x~0.2)1 MK (see, for example,/15/). The SXT has 5 metal-film filters (All200/~, A1MgMn, Mg, All2kt, and Be/16/) so that ratios of the signal seen through different filter pairs can be used to determine the equivalent isothermal temperature and emission measure of any stable feature by the same method as that used on the equivalent Skylab instruments/17,18/. The determination of electron temperature from broadband filter images is not a simple problem, because the response of the different filters has a similar functional form, making temperature diagnostics change only by a small factor in a given temperature interval. Strong et al. /19/ concluded that it would be hard to derive temperatures to a greater accuracy than 0.1 on a logarithmic scale from 1 to 10 MK. Thus an excellent knowledge of the instrument calibration, the relative response of the X-ray filters, good counting statistics, and a knowledge of the evolution of the source between the various exposures are required. An initial study of active regions by Hara et al. /20/ using SXT data has found that at about 2.7 MK the emission measure of an active region studied in detail (NOAA 6919) was 1.9 x 10~ cm 5 in the fainter loops and 2.8 x 1029 cm 5 in the core of the region. The core emission measure agrees well with similar measurements from XRP, outer region would not have been easily observed by XRP. However, Hara et al. also found evidence for a hightemperature component (Te > 5 MK) in these data. The emission measure of this component is less than 20% of that at 2.7 MK. At that level it seems a reasonable assumption that this is the high-temperature tail of a differential emission measure distribution and that the isothermal approximation is not valid to the 20% level even for active regions. Although there were some hints of such a high-temperature competent in the SMM data, the Bent Crystal Spectrometer (BCS) lacked sufficient sensitivity to determine its parameters reliably. The presence of a hightemperature component in active regions should be easily testable when the Sun has fewer active regions than at this stage of the cycle by the Bragg Crystal Spectrometer on Yohkoh (YBCS), which is nearly 10 time more sensitive than the BCS and has a lower-energy channel/21/. Hara et al. also made measurements in the quiet Sun and found temperatures of the order of 2.7 MK and and emission measure of 1.3 x 1026cm 5.

' SoftX-myTelescope

(4)9

Figure 6: Illustration of XBP flare and accompanying jet of hot coronal plasma that seems to flow along a loop at velocities in excess of 1000 km/s. This was observed on 7 December 1991.

X-RAY BRIGHT POINTS The uniform coverage with a relatively rapid cadence (256 s) of full-disk images at high sensitivity makes SXT almost ideal for observing XBPs. Harvey et o2./22/describe measurements of the lifetimes and spatial distribution of over 500 XRBs seen by SXT in June 1992. They find that the lifetimes vary from a few minutes to many days, with an average of about 12 hours (longer than a similar study based on S/cylab data by Golub et al./23/), However they also find a dual distribution of life times (as did Golub et O2.) with characteristic decay times of 11 hours and 37 hours (of. 8 and 35 for the SATIab results). It will remain to be seen if as the solar cycle decays whether the XBP lifetimes come more into agreement with the Slcylab values or whether this discrepancy represents a difference in the instrumental sensitivity and dynamic range. No particular trend XBPs location was found and the XBPs had similar average lifetimes in active region, quiet Sun and coronal hole areas. In an earlier study of the variability of XBPs, Strong et O2./6/found that there was a significant flare in about 10-20~ of XBPs which was often accompanied by the filling of a large loop-like structure with velocities of up to II00 km/s and over distances of several I00,000 km (see Figure 6). Nitta et 02./7/look at the correspondence of microwave images taken at 20 cm from the VLA and SXT images. For the most part there is a good correlation between the compact features seen in both, with the few discrepancies most likely coming from short-term evolution of either the radio or X-ray source. ,M,SR 14:44

(4)10

K.T. Strong and YohkohTeam

Figure 7: A series of SXT images showing the long term evolution of a coronal hole channel. Note how the channel maintains its basic shape over several rotations while developing slowly. It does not seem to share the expected differential rotation that the surface features have. CORONAL HOLES Coronal holes and coronal hole channels seem to persist for several rotations without significant distortion by differential rotation, see Figure 3. As fast reported by Hayashi et al. (private comm.) the structure of one such channel has remained unsheared over several rotations/24/. In Figure 7 (G. Linford, private comm.) the evolution of a particularly persistent coronal hole channel is followed over several rotations. The eastern boundary of the channel seems to be particularly sharp and high compared to the western boundary which is why it can be more clearly seen in the eastern hemisphere than in the west. It appears to be seen most clearly during the April and May rotations, after which the channel seems to slowly close or fill in with newly emerging flux. CONCLUSIONS After a year of operations Yohkoh continues m return fascinating new data, revealing a surprisingly dynamic corona. While we may have reasonably expected flaring active regions to exhibit such motions it is surprising to see that even the quiet Sun participates in the activity cycle, albeit at a reduced rate and intensity level. As the Sun slips towards solar minimum the focus of Yohkoh will shift to these type of quiet-Sun targets. From our initial sample of data this prospect is exciting as SXT will open new vistas for the solar community in quiet coronal studies. ACKNOWLEDGMENTS The Yohkoh team would like to thank the Institute of Space and Astronautical Science where Yohkoh operations

are centered and their staff is responsible for the mission and hosting the visiting scientists who make up the inter-

Soft X-rayTelescope

(4)11

national team, the National Aeronautics and Space Administration, the Naval Research Laboratory and the U.K. Science and Engineering Research Council help to fund several of the instruments that made up the Yohkoh payload. Many scientists have worked hard to make this mission the success it is, far too many to thank here so I will list the primary institutions: I.x~ckheed Solar & Astrophysics Laboratory, Mullard Space Science Laboratory, National Astronomical Observatory of Japan, Naval Research Laboratory, Rutherford Appleton Laboratory, Solar Physics Research Corporation, Stanford University, University of California - Berkeley, University of Hawaii, and University of Tokyo. KTS was supported by NASA contract NAS8-37334 with Marshall Space Flight Center and the Lockheed Independent Research Programme. REFERENCES 1. Vaiana, G., et al., 1973, Solar Phys., 32, 81. 2. Vaiana, G., et al., 1973, Astrophys. J. (Letters), 185, IA7 3. Krieger, A., et al., 1971, Solar Magnetic Fields, ed. R. Howard (IAU Symposium 43), 397. 4. Uchida, Y., et al., 1992, PubL Astron. Soc. Japan (Letters), Vol. 44, 5, L155, 1992. 5. Acton, L. W., 1992, Science, in press. 6. Strong, K. T., et al., 1992, Publ. Astron. Soc. Japan (Letters), Vol. 44, 5, L161, 1992. 7. Nitta, N., et al., 1992, Publ. Astron. Soc. Japan (Letters), Vol. 44, 5, L167, 1992. 8. Tsuneta, S., etal., 1992, Publ. Astron. Soc. Japan (Letters), Vol. 44, 5, L211, 1992. 9. Klimchuk, J., etal., 1992, Publ. Astron. Soc. Japan (Letters), Vol. 44, 5, L181, 1992. 10. Martens, P. C. H., and Gomez, D. O., 1992, Publ. Astron. Soc. Japan (Letters), Vol. 44, 5, L187, 1992. 11. MeAUister, A., et al., 1992, Publ. Astron. Soc. Japan (Letters), Vol. 44, 5, L205, 1992. 12. Shimizu, T., et al., 1992, Publ. Astron. Soc. Japan (Letters), Vol. 44, 5, L147, 1992. 13. Lin et al., 1984, Astrophys. J., 283, 421. 14. Pye et al., 1973, Solar Physics, $3, 417. 15. Saba, J. L. R., and Strong, K. T., Astrophys. J., 375, 789. 16. Tsuneta, S., et al., 1991, Solar Phys., 136, 37. 17. Vaiana, G. S., Krieger, A. S., and Timothy, A. F., 1973, Solar Phys., 53, 417. 18. Gerassimenko, M., and Nolte, J. T., 1978, Solar Phys., 60, 299. 19. Strong, K. T., etal., 1991, Adv. SpaceRes., 11, 1,117. 20. Hara, H., etal., 1992, Publ. Astron. Soc. Japan (Letters), Vol. 44, 5, L135, 1992. 21. Culhane, J. L., et al. 1991, Solar Phys., 136, 89. 22. Harvey, K., et al., this volume 1992 23. Golub, L., et al., 1976, Solar Phys., 49, 79. 24. Ogawara, Y., et al., 1992, in prep.